1. Field of the Invention
The present invention relates to an active matrix liquid crystal display comprising an active matrix substrate on which switching elements are formed in the shape of a matrix, a counter substrate opposed to the active matrix substrate and a liquid crystal layer therebetween.
2. Description of the Related Art
Liquid crystal displays have attracted attention as a display used instead of a CRT (cathode ray tube) due to their characteristics of saving space and low power. Among such liquid crystal displays, thin film transistor (hereinafter referred to as the "TFT") activated liquid crystal displays are superior because the liquid crystal therein responds at a high rate and images are displayed with high quality. Especially, TFTs made from amorphous silicon (hereinafter referred to as "a-Si") have been remarkably developed in recent years since films can be formed at a low temperature in such TFTs so that they can provide a more accurate and less expensive displays showing a larger picture.
FIG. 16 is a plan view showing an example of a conventional liquid crystal display using the a-Si in the TFTs. FIGS. 17 and 18 are sectional views taken on lines A-A' and B-B' of FIG. 16, respectively. The liquid crystal display comprises an active matrix substrate 122 on which TFTs 124 are formed in the shape of a matrix, a counter substrate 123 opposed to the active matrix substrate 122 and a liquid crystal layer 116 sandwiched therebetween. A structure of such a liquid crystal display will now be described in details based on its production steps as follows:
First, a metal than film is formed on a transparent insulating glass substrate 101 to form gate lines 102 as scanning lines and gate electrodes 102a branched from the gate lines 102. A SiN.sub.x film as a first insulating film 103, an a-Si layer as a semiconductor layer 104 of the TFTs 124 and another SiN.sub.x film as a second insulating film 105 are successively formed on the entire top surface of the glass substrate 101. The second insulating film 105 is then patterned as is shown in FIG. 16.
Next, after forming a metal thin film thereon, an n.sup.+ layer 106 doped with P is formed for making an ohmic contact with the a-Si layer 104. Then, source lines 107 as signal lines having a pattern as is shown in FIG. 16, source electrodes 107a branched from the source lines 107, drain electrode/shielding films 108 and shielding films 110 are respectively formed. The TFTs 124 each having a sectional structure as shown in FIG. 17 are formed near respective crossings of the gate electrodes 102a and source electrodes 107a.
Then, a third insulating film 112 is formed on the entire top surface of the resultant glass substrate 101, and contact holes 113 are formed in the third insulating film 112. A transparent conductive film is then formed so as to fill the contact holes 113 and partially cover the gate lines 102 to form pixel electrodes 114. An alignment layer 115 is further applied on the pixel electrodes 114, and then treated by a rubbing method. The active matrix substrate 122 is thus formed.
The counter substrate 123 is fabricated as follows: A color filter 120 is patterned on a transparent insulating glass substrate 121, if necessary. A transparent conductive film is formed over the entire top surface of the patterned substrate to form a counter electrode 118. An alignment layer 117 is then formed on the counter electrode 118, and is treated by the rubbing method.
The active matrix substrate 122 and the counter substrate 123 are then adhered to each other, and liquid crystal is injected therebetween to form the liquid crystal layer 116. The liquid crystal display is fabricated in this manner.
In such a liquid crystal display, the drain electrode/shielding films 108 serving as both the drain electrodes and the shielding films as well as the shielding films 110 are provided above the gate lines 102 as is shown in FIG. 18 for the following reason: In obtaining high accuracy by increasing the number of pixels in a unit area, 1-Horizontal inversion driving for inverting polarity of signals applied to the pixel electrodes 114 by gate line 102 is adopted, because more pixels decrease the interval between the adjacent pixels. In this case, an interaction is caused between the adjacent pixel electrodes 114 to disturb the electric field over the gate lines 102. As a result, the liquid crystal particles are also disturbed, and, for example, light is leaked while displaying black in the normally white mode of the liquid crystal, resulting in degraded contrast. The shielding films 108 and 110 are formed above the gate lines 102 to shield the light leak, thereby preventing the degradation of the contrast.
A system in which the shielding films are formed above the source lines of the active matrix substrate is also known (M. Tsumura et al., "High-Resolution 10.3-in. Diagonal Multicolor TFT-LCD", SID 91 DIGEST, pp. 215-218). In this system, contrast is degraded due to light leak caused by the disturbed electric field on the gate lines in the 1-Horizontal inversion driving as mentioned above. In the 1-Vertical inversion driving for inverting polarity of signals applied to the pixel electrodes by source lane, the degradation of contrast can be avoided because the electric field is disturbed above the source lines. However, in this system, an IC for driving has a disadvantageously large load because it is impossible to drive the counter electrode on the counter substrate so as to aid the application of signals to the pixels.
For the reasons described above, the shielding films 108 and 110 are formed above the gate lines 102 in the conventional liquid crystal displays.
In such conventional liquid crystal displays, as is evident from FIGS. 17 and 18, a parasitic capacitance is generated in overlapped portions between the drain electrode/shielding films 108 and gate lines 102, shielding films 110 and the gate lines 102, or the pixel electrodes 114 and the gate lanes 102, thereby degrading display characteristics. An effect of the parasitic capacitance on the display characteristics is represented by a ratio obtained by the following equation (I): ##EQU1## Since the conventional liquid crystal has no storage capacitance, the parasitic capacitance largely affects display characteristics. However, in order to provide a storage capacitance with each pixel to solve this problem, lines and the like for storage capacitance should be formed, resulting in reducing the aperture ratio. Furthermore, when the lines for the storage capacitance are formed from a transparent conductive material so as not to reduce the aperture ratio, the yield is degraded because of the additional production steps.